The performance-stability conundrum of BTP-based organic solar cells

doi:10.1016/j.joule.2021.06.006

Joule

Volume 5, Issue 8, 18 August 2021, Pages 2129-2147

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Highlights

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BTP-based OSCs (including BTP-4F) are intrinsically unstable or have low performance
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Molecular designs that yield materials with high Tg are needed to get long-term stability
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Additives in ink formulations that reduce the mobility of BTP-based NFAs are required

Context & scale

Organic solar cells (OSCs) have attracted considerable attention for their promise of large-scale processing. Particularly, OSCs based on non-fullerene acceptors have enjoyed significant attention due to dramatically improving efficiency, however, long-term stability is crucial and, therefore, becomes an imperative research goal in the field. Here, we determine the T_g of seven BTP-based non-fullerene acceptors. Using the Gordon-Tayler relations, we have developed a structure-T_g framework that can disentangle the contribution to T_g of the outer and inner sidechains, as well as the two different cores. We also show that PC₇₁BM has miscibility above the percolation threshold in PM6 and can, thus, maintain local charge percolation. However, PC₇₁BM is not miscible with BTP-C3-4F and unfavorable vertical gradients that still degrades performance. A high T_g component with suitable electronic structures that prevents BTP diffusion would be required to achieve commercial viability.

Summary

As the power conversion efficiency of organic photovoltaic has been dramatically improved to over 18%, achieving long-term stability is now crucial for applications of this promising photovoltaic technology. Among the high-efficiency systems, most are using BTP-4F and its analogs as acceptors. Herein, we determine the thermal transition temperatures (T_g) of seven BTP analogs to develop a structure-T_g framework. Our results point out an unresolved molecular design conundrum on how to simultaneously achieve high performance and intrinsic stability with BTP-based acceptors. We also show that PC₇₁BM has miscibility above the percolation threshold in PM6 and can maintain local charge percolation and improved stability in ternary devices. However, PC₇₁BM is not miscible with BTP-C3-4F and unfavorable vertical gradients that develop during aging still degrade performance. This points to a second thermodynamic conundrum. A compound with differential miscibility in the donor polymer can only impact percolation, and a compound with differential miscibility with the BTP only impacts diffusion.

Graphical abstract

Keywords

organic solar cells

stability

glass transition temperature

miscibility

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